Two Kapiti fishermen who rescued a drowning woman from the sea at first mistook her for one of their crab pots.

Waikanae Crab owner Matt Whittaker and his son Jesse were fishing about 300 metres off Paraparaumu Beach on Sunday morning when Jesse thought he spotted one of their pots 200 metres further out to sea.

"We saw a flicker of light on something, it looked like a pot. I wondered how the hell it had drifted out so far," Matt said.

"We were going to keep going in the other direction, but decided to go and have a look."

As they got closer, Jesse saw a spout of water coming from the object and he yelled out: "Dad, I think it's a human being."

When the pair got alongside, they saw it was a woman, partly unconscious, going up and down under the water.

"She was right in the channel, in the current," Matt said. "She did not see us, she was just away with it. I yelled out: 'Are you all right?' She turned and fixed her eyes on Jesse but could not speak.

"Her lips were blue/purple, she was in the last dire struggle to live," Jesse said.

They pulled her onto their boat, wrapped her in everything warm they had and put her in the recovery position, Jesse supporting her on his knees as she convulsed and vomited seawater.

"I held her hand, told her to keep breathing, she was going to be all right. She was shaking so much she jumped off my knees, it was really scary, really intense. I could hear the water bubbling in her lungs. It freaked me out pretty hard."

The woman uttered the word "children" over and over again.

Issuing a mayday call, Matt told Wellington Maritime Radio a search was needed.

"Because she was so far out we thought she had come off a boat and there were children in the water."

Kapiti Charter deviated from its course to Kapiti Island and started searching the waters.

When the fishermen got the woman to shore, police and an ambulance were waiting. Police ascertained her three children, aged 16, 11 and 7, were safe at home.

The woman, 46, was taken to Wellington Hospital and admitted to a psychiatric unit.

She said she had been walking her dog on the beach, tried to save it from the waves and was pulled out to sea by strong currents.

Police had been told earlier that a dog was guarding a handbag on the beach. They found no identification in the handbag and got a call about the near-drowning while they were on the beach.

Detective Sergeant James McKay praised the men's actions.

"She was suffering from extreme hypothermia. She would definitely have drowned if they had not saved her."

Matt Whittaker was a reluctant hero, saying he had never found a person in the middle of the ocean before.

"You just do what you have to do. We are just really pleased she's alive."